I was thinking Jean should return, evolve into the Phoenix Force and the F4 encounter her in space. She mutates them similar to Magneto's machine...tying in the scene from X1...

Linking X-Men to F4 via the Phoenix is an intriguing idea that has potential. The MCU fanboys would go crazy, of course, even though they can accept 'fake Mandarin' and an Ultron who will have nothing to do with Ant-Man in a film called Age of Ultron that has nothing to do with the comic called Age of Ultron.

But it's a nice way of tying the two series together.

I'm still not convinced an F4 reboot is going to be a big hit, especially with Fox's poor marketing of these films.